You said that your snakes ate rats before so he prolly comes out when he smells the rat, realizes that he doesnt really like the rats anymore and prefers mice instead. It would be like if u smelle something cooking and came into the kitchen only to realize that what u smelled was actually turkey and not chicken. You couldnt smell exactly what it was before but you realized once u got into the kitchen. I really dont know how acute a bp's sense of smell is exactly so maybe he just smells/senses rodent and comes running out but then realizes its a rat so he goes back inside. You said tht his queu is taking out the water dish so he prolly sees you do that, gets ready, then realizes its not what he really wants and goes back inside.
This is just my 2 cents, im not claiming to be a know it all lol.